1. Field of the Invention
This invention relates to the structure of a lens mounting of relatively large size capable of attaching to a television camera, single lens reflex camera or video camera, and more particularly to improvements of the cam mechanism for controlling movement of lenses along an optical axis of the photographic optical system.
2. Description of the Related Art
In the zoom lenses of the mechanical compensation type, it has been common in the art that the movement of the movable lens units is controlled by the camming slits of the cam sleeve. In other words, the guide pins planted on the holding rings of the movable lens units engage in the linear camming slits provided in the fixed tube in parallel to the optical axis and the non-linear camming slits of the cam sleeve so that as the cam sleeve rotates about the optical axis, the guide pins move in the camming slits.
Particularly in the lens mounting of relatively small size such as the interchangeable one for 8 mm cine cameras or single lens reflex cameras, the cam sleeve in many cases is provided with a plurality of camming slits.
On the other hand, a sectional view of the conventional lens mounting of relatively large size for television cameras is shown in FIGS. 5 and 6. For note, FIG. 6 is a cross-sectional view taken along line C--C of FIG. 5. 201 is a magnification varying lens system, and 202 is a compensating lens system. 101 is a focusing sleeve; 102 is a fixed barrel; and 103 is a cam sleeve connected with a zoom actuating ring 110 by a connection pin 109 to rotate, and provided with camming grooves, one for each of holding members 105 and 108, to axially move the magnification varying lens system (variator) 201 and the compensating lens system (compensator) 202 through the holding members 105 and 108. Also, in order to guide the axial movement of the holding members 105 and 108 while restraining them from rotation in accompaniment with rotation of the cam sleeve 103, there are provided a round bar 104 and a round bar 106, both of which are fixed to the fixed barrel 102. The round bar 104 and the holding member 105 engage each other along a certain length, l, in the axial direction. Meanwhile, the round bar 106 is positioned in almost 180.degree. spaced relation to the round bar 104. This round bar 106 generally has a function that restrains it from rotation about the optical axis and makes it engage the holding member 105 with a radial looseness to achieve smoothness of movement of the latter.
However, the zoom lens of this type had a drawback that when in use at an inclined angle, or with the front lens unit pointed either upward or downward, an image shift in the longitudinal or lateral direction was caused to occur, because the holding member 105 holding the variator 201 and the holding member 108 holding the compensator 202 were tilted.
The reason why the holding members tilt is that there is need to provide a minute gap (.DELTA.x) between the round bar 104 and a part 105a of the holding member 105 which engage each other, while the lower engagement of the round bar 106 and the holding member 105 is free in the radial directions. Therefore, when the lens is tilted with the front member downward or upward, the variator 201 and compensator 202 incline to an angle .theta.=tan.sup.-1 (.DELTA.x/l), because it is fulcrumed mainly about a forked portion 105b or so-called one-sidedly held. In this case, to reduce this angle, there is a method of lessening the gap .DELTA.X between the round bar 104 and the holding member 105. But, if it is too much done, the required driving power becomes very large. So, the zooming becomes very difficult to perform. Another method is to increase the length, l, of engagement of the round bar 104 and the holding member 105. But, this leads to increase the size of the lens barrel. Also, because there is a limit on the size of the round bar 104 in the longitudinal direction, too much an increase of the length is not desirable.